Endodontic instruments made from alloys including Nickel Titanium are desirable because of various physical characteristics of such alloys. When heat treated, such alloys take on even more unique physical characteristics including, in some cases, limited memory wherein an instrument will partially rebound to its initial configuration after undergoing forced mechanical deformation.
Salt baths or salt solution technology at high temperatures (e.g., about 500° C.) have been used in stress-induced heating of endodontic blanks to bring such blanks to 100% austenite phase as discussed, for example, in U.S. Pat. No. 6,783,438 entitled “Method of Manufacturing an Endodontic Instrument” to Aloise et al. Such use of salt baths has been described as undesirable, however, in the U.S. Pat. No. 6,783,438 patent as well as, for example, U.S. Patent Publication Number 2010/0233648 entitled “Endodontic Instrument and Method of Manufacturing” to McSpadden et al. because of concerns of corrosion to the tooling or other materials involved in such treatments. For situations in which salt baths have been used to heat treat an endodontic blank, the exposure of the blank to the bath has been rapid, measured on scale of seconds as opposed to hours as taught, for example, in U.S. Pat. No. 6,149,501 entitled “Superelastic Endodontic Instrument, Method of Manufacture, and Apparatus Therefor” to Farzin-Nia et al.
In all of the examples given above, endodontic blanks (as opposed to finished endodontic instruments) were heat treated to acquire super elastic properties and then later cut to make finished endodontic instruments with cutting surfaces. Thus, the nature of the treatments in the prior examples included very limited treatment periods and specific timing regarding the phase of the overall manufacturing process in which heat treatment occurred.